On 11/11/2016 12:18 PM, Michael Williamson wrote:
That still requires the repetition of the name of the attribute, which
I'd rather avoid. I've put together a variation on hybrid_property
which automatically assigns the label by scanning through the class
dict. It could probably do with a bit of memoization and and proper
handling of subtyping (i.e. checking __dict__ of supertypes), but does
this seem sane to you? Is there a neater way?
class hybrid_property2(hybrid_property):
def __get__(self, instance, owner):
if instance is None:
expression = self.expr(owner)
for key, value in owner.__dict__.items():
if value == self:
return expression.label(key)
return expression
else:
return self.fget(instance)
Thanks for the suggestions so far, Mike and Simon.
Getting the name of the attribute you're set on is not something I know
that there's any way to do without the kind of thing you're doing.
Decorators usually work out because fn.__name__ is there to give us that
info.
Is there a sensible way to do this? Or am I going about the problem
the wrong way? In practice, I'm using this to define properties
backed by a JSON field. In case it makes any different, here's the
actual use-case I have:
from __future__ import unicode_literals
import os
from sqlalchemy import create_engine, Column, Integer
from sqlalchemy.dialects.postgresql import JSONB
from sqlalchemy.event import listens_for
from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Session
from sqlalchemy.pool import StaticPool
def json_property(json_column, name, type_=None):
def instance_get(self):
return getattr(self, json_column.key)[name]
def instance_set(self, value):
json_obj = getattr(self, json_column.key)
if json_obj is None:
setattr(self, json_column.key, {})
getattr(self, json_column.key)[name] = value
def cls_get(cls):
expression = json_column[name]
if type_:
return expression.astext.cast(type_)
else:
return expression
return hybrid_property(fget=instance_get, expr=cls_get,
fset=instance_set)
Base = declarative_base()
class Person(Base):
__tablename__ = "person"
id = Column(Integer, primary_key=True)
data = Column(JSONB, nullable=False)
born = json_property(data, "born", Integer())
engine = create_engine(os.environ["TEST_DATABASE"],
poolclass=StaticPool) engine.execute("SET search_path TO pg_temp")
@listens_for(engine, "engine_connect")
def set_search_path(connection, branch):
connection.execute("SET search_path TO pg_temp")
Base.metadata.create_all(engine)
session = Session(engine)
session.add(Person(born=1881))
session.commit()
assert session.query(Person.born).one() == (1881, ) # Works fine
assert session.query(Person.born).one().born == 1881 # Raises
AttributeError since hybrid_property uses instance_get as the name
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